The present invention is generally related to decimation circuits used, for example, in oversampled delta-sigma analog-to-digital converters, and, in particular, to a decimation circuit and method for filtering quantized electrical signals while providing phase angle correction with a substantially linear phase response at least over a desired passband range.
Data acquisition systems for generating digital data for the purposes of computation may receive analog input signals from a plurality of sensors, such as voltage and current sensors, each having a respective phase angle characteristic. The respective analog signals from the plurality of sensors must be digitized or quantized before they can be used by a computer as a basis for supporting computations. It is desirable to include respective analog-to-digital converters within the confines of an inexpensive single monolithic integrated circuit. Such data acquisition circuit can be constructed using metal-oxide-semiconductor (MOS) integrated circuit technology and is suited for applications such as power metering, electric motor and internal-combustion engine control.
Oversampling analog-to-digital converters of delta-sigma type are particularly economical of digital hardware. The use of such converter introduces the need for decimation filters such as sinc.sup.k decimation filters, in which the kernel is a sampled-data representation of a suitable time-domain response, to achieve sufficient selectivity against harmonic components of the sinusoid being filtered. For example, for k=1 the time domain response corresponds to a rectangular time response, while for k=2, the time domain response corresponds to a triangular time response. In each case, phase angle differences (due to each of the sensors respective phase angle characteristic) among the analog signals respectively acquired with the plurality of sensors are propagated in the respective output signals from the decimation filter. The phase angle differences usually require correction or equalization before further signal processing is performed. In addition, the phase angle correction must be accomplished in a manner consistent with providing a substantially linear phase response over a bandpass range of interest. To reduce the effects of such phase angle differences, sensors are constructed in accordance with tight specifications which generally result in higher than desired sensor cost. Thus it is desirable to provide a decimation circuit capable of providing phase angle correction while operating in a manner consistent with providing a substantially linear phase response over the bandpass range of interest.